Light emitting diodes (LEDs) have been used as light sources of various kinds of electric apparatuses such as a display device, an illumination apparatus and so on because they can emit a high brightness light with low power consumption. With recent practical use of blue (B) LEDs in addition to red (R) and green (G) LEDs, various luminous colors have been achieved by combining these RGB LEDs together or combining LEDs with phosphors for converting a wavelength of a light emitted from the LEDs.
In addition, there has been known a light emitting device which produces a white light by mixing a blue light directly emitted from a blue LED and a yellow light produced by colliding a blue light with a phosphor. In the light emitting device producing the white light as described above, a YAG-based phosphor, for example, is dispersed in a resin material, such as a silicone resin or the like, which seals a blue LED. The YAG-based phosphor converts a wavelength of the blue light emitted from the blue LED into a wavelength of the yellow light.
However, in case where the white light is produced by mixing the blue light and the yellow light, chromaticity coordinates of the mixed lights vary in all directions for each manufactured object if a full width at half maximum of an emission wavelength or a peak wavelength varies, or a concentration of the phosphor contained in the resin material is uneven due to a manufacturing tolerance of the blue LED.
For example, as shown in FIG. 6, a yellow phosphor (chromaticity coordinate Y(xY, yY)) and a red phosphor (chromaticity coordinate R (xR, yR)) are combined to prepare a phosphor layer of a chromaticity coordinate RY(xRY, yRY). In addition, the phosphor layer and a blue LED chip (chromaticity coordinate B(xB, yB)) are combined to produce a color temperature of 3500K. In this case, a color temperature line set at 5000K of white by the blue LED chip and the yellow phosphor is not in parallel with an isotemperature line (with an angle θ) of 3500K, which results in a deviation in the color temperature.
In recent years, since light emitting devices using LEDs, e.g., a light source for an illumination apparatus, have been frequently used in combination of several of them, there is a need to minimize a manufacturing deviation in a color temperature for each light emitting device.
As one of conventional techniques for reducing color temperature deviations, there has been known a white light emitting device which reduces a color temperature deviation by classifying ranks of blue LEDs based on their emission wavelengths and luminances, and adjusting luminances of phosphors to correspond to the ranked LEDs (see, e.g., Japanese Patent No. 4201167). In this technique, the blue LED and the phosphor are appropriately combined together such that chromaticity coordinate of a light emitted from the blue LED and chromaticity coordinate of a light emitted from the phosphor are plotted on an isotemperature line at desired color temperature. In this way, a chromaticity coordinate deviation in the mixed light is made to lie on the isotemperature line. The isotemperature line refers to a line on a chromaticity diagram on which chromaticity coordinates providing the same color temperature are plotted. If a chromaticity coordinate deviation lies on the isotemperature line, a color temperature deviation is decreased.
Further, there has been known an LED light source including a color temperature adjusting member containing a phosphor (second phosphor) which emits a light of a color different from a color of a light emitted from a phosphor (first phosphor) which emits a yellow light (see, e.g., Japanese Patent Application Publication No. 2009-231569). In this LED light source, even if there is a deviation in an LED emission wavelength, concentration of the first phosphor, and so on, the second phosphor can be used to arbitrarily set a chromaticity coordinate of a final light emitted from the color temperature adjusting member.
The techniques described above may reduce a color temperature deviation of a monochromatic light and perform color temperature adjustment. However, these techniques still have a problem that it is difficult to make the 5000K color temperature line of a white light by the blue LED chip and the yellow phosphor parallel to the 3500K isotemperature line (with the angle θ), which may result in difficulty in implementing desired color temperature.